1. Field of the Invention
The present invention relates to a self-oscillation system for driving a linear oscillatory actuator at its resonant frequency, and more particularly to the system added with a controller for keeping a consistent oscillation even under a varying load condition.
2. Description of the Prior Art
A linear oscillatory actuator has been utilized in many electrical devices such as a reciprocatory shaver, and normally comprises a stator carrying a winding and a reciprocator carrying a permanent magnet. The winding is connected to receive a periodical electric current from a power source to generate a magnetic flux which interacts with the permanent magnet to cause a mechanical resonance of the reciprocator, thereby forcing the reciprocator to move linearly relative to the stator.
U.S. Pat. No. 6,133,701 discloses a system for driving the linear oscillatory actuator with a use of a self-oscillation circuit. The circuit is connected to receive a back electromotive force developed across the winding in a positive feedback manner to generate a drive pulse by which the electric current is periodically supplied to the winding for continuing the mechanical resonance of the reciprocator. With this scheme, however, the reciprocator is difficult to keep the consistent oscillation without being considerably damped when subjected to a heavy load. In order to alleviate this problem, a PWM controller could be added to the circuit for varying a pulse width of the drive pulse in accordance with an instant oscillation amplitude of the reciprocator, as disclosed in U.S. Pat. No. 5,632,087. The patent discloses the use of an external sensor disposed in juxtaposition of the reciprocator for detection of the instant oscillation amplitude of the reciprocator. In addition to increasing the number of discrete components, the external sensor requires a complex and cumbersome design of turning the sensor output to the PWM controller for precise PWM control based upon the true oscillation amplitude of the reciprocator. This is particularly true when adding the PWM control to the self-oscillation circuit giving the drive pulse in the positive feedback of the back electromotive force.
The present invention has been accomplished in view of the above problem to provide an improved self-oscillation system for driving a linear oscillatory actuator around its resonant frequency. The linear oscillatory actuator includes a reciprocator which moves linearly relative to a stator around its natural mechanical resonance frequency. One of the stator and the reciprocator carries a winding, while the other of the stator and the reciprocator carries a permanent magnet. The winding is connected to receive a periodical electric current to develop a magnetic flux which interacts with the permanent magnet to continue driving the reciprocator around its natural frequency. The system of the present invention includes a self-oscillation circuit which is connected to receive a back electromotive force (Vbemf) appearing across the winding in a positive feedback manner to generate a drive pulse by which the electric current is periodically supplied to the winding for continuing the oscillation of the reciprocator. In order to regulate the electric current in match with a load applied to the reciprocator, the system includes a detector circuit which monitors an instant oscillation amplitude of the reciprocator and provides a detector output indicative of the instant oscillation amplitude. Also, a controller is included to give a control signal indicative of the detector output to the self-oscillation circuit so that the self-oscillation circuit modulates the drive pulse in accordance with the control signal to regulate an electric power being supplied to the winding, thereby compensating for a variation in the load being applied to the reciprocator so as to continue the resonant oscillation of the reciprocator. The characterizing feature of the present invention resides in that the detector circuit is connected to monitor Vbemf of the winding in the absence of the electric current and to judge the instant oscillation amplitude of the reciprocator based upon the monitored Vbemf. Accordingly, the system of the present invention can best utilize Vbemf of the winding for accurate detection of the instant oscillation amplitude of the reciprocator without requiring an external sensor, and therefore assure a reliable feedback control of keeping the consistent oscillation amplitude around a constant level, i.e., without considerably damping the resonance oscillation under a varying load condition, which is therefore a primary object of the present invention.
In a preferred version of the present invention, the self-oscillation circuit comprises a voltage amplifier providing the drive pulse of a variable pulse width, and a switch connected tin series with the winding across the power source to flow the electric current periodically from the power source through the winding. The voltage amplifier is connected to receive Vbemf through a positive feedback path from the linear actuator to provide the drive pulse which is applied through the switch to periodically flow the electric current through the winding for continued oscillation of the reciprocator. The switch allows the electric current to be self-regulated to vary in a reverse proportion to Vbemf, thereby forming a self-current compensator which compensates for an amplitude of reciprocator reduction resulting from the increasing load. The voltage amplifier also receives the control signal form the controller to act as a pulse-width modulator which varies the pulse width of the drive pulse by the control signal in a direction of increasing the pulse width with the decreasing Vbemf detected at the detector circuit. The switch responds to the drive pulse to flow the electric current over a period of the drive pulse width in compensation for a variation in Vbemf indicative of the load applied to the reciprocator for continued resonant oscillation of the reciprocator. In this manner, the PMW control can be active in synergy with the self-current compensator to compensate for the varying load over a wide range.
The power source is preferably a DC power source which supplies the DC electric current to the winding through the switch, and the detector circuit can be designed to judge the oscillation amplitude based upon the peak voltage of Vbemf.
In another preferred version of the present invention, the self-oscillation circuit comprises a voltage amplifier providing a reference signal of a frequency synchronized with the oscillation of reciprocator, an AND circuit having one input receiving the reference signal and the other input receiving the control signal from the controller, and a switch connected in series with the winding across the power source to flow the electric current from the power source through the winding. The controller comprises a sawtooth generator which issues a sawtooth signal having a frequency synchronized with the oscillation of the reciprocator, and a comparator which compares the sawtooth signal with the detector output indicative of the instant oscillation amplitude of the reciprocator in order to provide the control signal in the form of a rectangular pulse of which width varies in reverse proportion to Vbemf detected at the detector circuit and therefore in direction proportion to the load being applied to the reciprocator. The AND circuit acts as a pulse width modulator to modulate the reference signal by the control signal so as to provide the drive pulse of which pulse width varies in direct proportion to the load. The switch responds to the drive pulse to flow the electric current periodically over a period of the drive pulse width in compensation for a variation in Vbemf indicative of the load applied to the reciprocator for continued resonant oscillation of the reciprocator. Also in this instance, the switch allows the electric current to be-self-regulated to vary in a reverse proportion to Vbemf, thereby forming a self-current compensator which compensates for an amplitude of reciprocator reduction resulting from the increased load. Thus, the PMW control is made available in synergy with the self-current compensator to compensate for the wide range load variation. Further, since the sawtooth signal is derived from the reference signal reflecting Vbemf responsible for generating the drive pulse, the resulting control signal can be accurately tuned to the drive pulse, enabling to make a reliable PWM control for keeping the oscillation amplitude of the reciprocator around a constant level. In this connection, the sawtooth generator includes a rectangular wave generator which receives the reference signal from the voltage amplifier and reshapes it into a rectangular ware signal, and an integrator which integrates the rectangular wave signal to provide the sawtooth signal of which frequency is synchronized with the oscillation of the reciprocator.
Further, the detector circuit comprises a differential amplifier providing an output voltage indicative of a difference between Vbemf and a reference voltage, and a peak voltage detector which determines a peak voltage of the output voltage as the detector output which is indicative of the oscillation amplitude of the reciprocator, and is compared with the sawtooth signal at the comparator to provide the rectangular control signal. Thus, Vbemf well indicative of the instant oscillation amplitude of the reciprocator can be suitably processed to provide the control signal which compensates for the load variation that the reciprocator experiences during its oscillation.
These and still other objects and advantageous features of the present invention will become more apparent from the following description of the preferred embodiments when taken in conjunction with the attached drawings.